1. Field of the Invention
The present invention relates in general to a heat-resistant vibration damping rubber composition, and more particularly to a heat-resistant vibration damping rubber composition which is formed by using an ethylene-propylene-diene rubber (EPDM) material and which exhibits not only excellent thermal resistance properties (thermal stability), but also durability and vibration damping characteristics that are both improved over conventional heat-resistant vibration damping rubber compositions.
2. Discussion of Related Art
In automotive roadway vehicles or railway vehicles, a vibration damping rubber structure of various types is conventionally interposed between two members of a vibration or shock transmitting system, for the purpose of preventing transmission of the vibration or shock to rigid components or members.
Such vibration damping rubber structures as described above, engine mounts used on automobiles, for instance, principally use natural rubbers (NR), or blends or mixtures of natural rubbers (NR) and synthetic rubbers such as styrene-butadiene rubbers (SBR) or butadiene rubbers (BR), since the natural rubbers and such mixtures are excellent in vibration damping characteristics and durability.
In recent years, there have been increasing demands for reducing fuel consumption and generation of noises externally of the vehicles, and purifying exhaust emissions. Accordingly, there has been a tendency toward an increase in the temperature within an engine room, so that the vibration damping rubber structures used as the engine mounts tend to be exposed to a comparatively high-temperature environment. Therefore, the vibration damping rubber structures disposed within or adjacent to the engine room are required to exhibit improved heat resistance properties. However, the conventional vibration damping rubber structures using a natural rubber (NR) or a mixture of the natural rubber and a synthetic rubber as described above suffer from drawbacks such as deterioration of physical properties and relatively early creep, due to exposure to the high-temperature environment, since the natural rubber (NR) which is the principal rubber component of the rubber composition has an insufficient degree of heat resistance. Accordingly, it has been considerably difficult to manufacture a vibration damping rubber structure which exhibits satisfactory thermal resistance properties.
In view of the drawbacks of the conventional vibration damping rubber structures indicated above, there have recently been proposed to use various kinds of vibration damping rubber composition which use known synthetic rubbers having a high degree of thermal resistance, such as ethylene-propylene-diene rubber (EPDM), in place of the natural rubber (NR) which has been used as a rubber material having excellent durability and vibration damping characteristics.
Although the EPDM material has excellent thermal resistance properties, this material which has a comparatively low molecular weight has a low degree of creep resistance and a high degree of dynamic spring stiffness, and has difficulty to give the vibration damping rubber composition sufficient durability and excellent vibration damping characteristics required for the vibration damping rubber structure. In an effort to lower the dynamic spring stiffness and improve the durability of the vibration damping rubber composition using the EPDM material, there have been proposed various approaches, for example, an attempt to increase the molecular weight of the EPDM material to an extent possible to avoid an adverse influence on the mixing and formability of the rubber composition, and an attempt to add as a reinforcing agent a high-structure carbon black. However, none of such approaches permits the rubber composition using the EPDM material to exhibit the durability and dynamic spring stiffness which are comparable with those of the known rubber composition using the natural rubber (NR). Thus, the known vibration damping rubber composition using the EPDM material still suffers from some problems such as insufficiency of an vibration damping effect achieved by the vibration damping rubber structure.